<!DOCTYPE html>
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   <head>
     <title>mo@movablelimits</title>
     <meta charset="utf-8"/>
     <script type="text/javascript">
       function doTest() {
         document.getElementById('a').removeAttribute('movablelimits');
         document.documentElement.removeAttribute("class");
       }
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    </head>
    <body>
    <math>
      <munder>
          <mo id="a" movablelimits="false">∑</mo>
          <mi>x</mi>
      </munder>
    </math>
    </body>
</html>